8 minutes=480 seconds.
If n is the number of times you run and t seconds is the time to run the distance, a linear relationship would be t=480-20n. From this equation, which graphs as a straight line, when n=480/20=24, t would be zero, so n=24 is the x-intercept and the y-intercept is 480 seconds (8 min) which is the starting point in time. The graph is the segment joining the intercepts. The model is clearly limited because no-one can run any distance in zero time. And neither t nor n can be negative.
(A better graphing model may be to consider 20 seconds as a fraction of the current running time, so that is 20/480=1/24 or about a 4% reduction in time. The mathematical model would be t=480(23/24)ⁿ. When n=1, t=460 seconds, 20 seconds short of 8 minutes. But when n=2, t=440.83 seconds, which is less than 20 seconds shorter. When n=24, t=172.8 secs approx. The graph is exponential and t is never zero, although it approaches zero when n is very large.)